1. Field of the Invention
The present invention relates to fluid shear couplings, and particularly to a coupling which provides for several distinct operating speeds.
2. Description of the Prior Art
Fluid shear couplings, such as those typically used as fan drives for vehicle engines, are well known in the art. Such couplings typically include a rotary drive disc which is driven by the engine, and which is rotatably mounted within a housing defined by a driven member. A quantity of viscous, shear liquid is admitted from a reservoir chamber to a fluid shear chamber defined by a relatively close spacing of portions of the drive disc and driven member housing. Depending upon the amount of shear liquid in the fluid shear chamber, the degree of rotary coupling between the drive member and the driven member is varied. In general, such devices lower the power loss to the radiator cooling fan or other driven component by correlating the power requirements of the driven component with the engine cooling requirement at various engine speeds and ambient temperatures.
The variance in coupling, resulting from the variation in shear fluid within the shear chamber, is usually controlled by a temperature responsive valve assembly. The valve opens to admit a larger quantity of fluid into the shear chamber when high cooling requirements are present. Closing of the valve results in a limitation of the shear fluid within the shear chamber and consequently presents a reduction of rotary coupling between the members. Such assemblies often include a passageway for the shear liquid to move from the radially outermost portion of the fluid shear chamber to the reservoir. The shear liquid is deflected so as to flow from the radially outermost part of the fluid shear chamber through the passageway and then to the reservoir chamber.
It has been the practice in the prior art to provide annular grooves in the driven member adjacent the outer perimeter of the drive member to produce the pumping action required for returning the shear fluid to the reservoir. The passageway leading to the reservoir opens into the annular groove, and a dam is positioned in the groove adjacent the passageway opening and the increased pressure caused by the fluid impacting the dam enhances the pumping action. In U.S. Pat. No. 3,856,122, issued to Leichliter on Dec. 24, 1974, there is disclosed a viscous fluid shear coupling having two opposed, arcuate channels and associated opposed holes within the channels at the same radial location outward of the shear chamber. Similar devices which include only a single channel and associated passageway opening are disclosed in U.S. Pat. Nos. 4,007,819, issued to Maci on Feb. 15, 1977; 4,004,668, issued to Blair on Jan. 25, 1977; and, 3,809,197, issued to Clancey on May 7, 1974.
In U.S. Pat. No. 3,174,600, issued to Oldberg on Mar. 23, 1965, there is disclosed a temperature-responsive fluid shear coupling which includes a rotatable member with a tangentially directed passageway opening into an annulus positioned outward of the center drive disc. The facing of the rotatable member against the disc rotation forces fluid out of the shear chamber into the reservoir, whereas directing the rotatable member in the opposite direction causes the fluid to be drawn into the annulus.
Viscous drives characteristically operate only in the fully engaged or disengaged modes. The normal drive design incorporates a fluid storage reservoir near the radial center of the drive body. This fluid is throttled to the working chamber of the drive by a thermally sensitive valve assembly as a result of changes in ambient temperature. This type of drive also incorporates the fluid pumping system which is able to return the shear fluid to the storage chamber when drive engagement is not required. The pump-back system most typically is defined by the described arrangement of an annulus near the outside diameter of the shear chamber with a fluid dam and drain-back passage located at the lagging end of the dam. Functionally, the fluid introduction valve allows fluid flow rates into the drive proportional to ambient temperature. When the entering fluid rate exceeds the pumping system's ability to remove it, i.e. at the drive engagement temperature, filling of the drive or shear chamber commences. Because the pumping system loses efficiency dramatically as the difference between input and output speed diminishes, the partially engaged condition for the typical drive is highly unstable and the unit characteristically passes directly from disengagement to full engagement.